Today’s colloquium paper is “An Empirical Argument for Substance Dualism” by Perry Hendricks. Hendricks is a graduate student in philosophy at Trinity Western University in British Columbia, where he also received his BA. His interests include philosophy of mind, philosophy of religion, and epistemology.

An Empirical Argument for Substance Dualism

Perry Hendricks

A common problem with arguments for dualism is that they rely on modal premises that are only supported by dubious intuitions. This results in the arguments having a narrow scope—only those who already hold the needed intuitions will find them to be convincing. In this paper, I try to remedy this situation by constructing a new modal argument whose key premise is empirically supported. I begin by formulating the physicalist thesis and make clear its commitments. Next I explicate the notions of reduction and substance. After this, I argue that Twin Earth—a physical duplicate of Earth (including its history and its inhabitants) is possible and that this possibility is empirically supported. I finish by showing that the possibility of Twin Earth entails that selves cannot be reduced and are not supervenient, and this entails that they are non-physical. Further, since selves are substances, it follows that substance dualism is true.

It is not uncommon to hear the argument that if there is an afterlife, then dualism must be true. However, dualism is false, and hence there is not an afterlife. It is also not uncommon to hear the argument that if dualism is true, then the probability of theism rises. I find neither of these theses compelling—I think that physicalism is compatible with an afterlife and that dualism does not raise the probability of theism—but if my argument is correct, it will provide a way to circumvent the first argument while providing support for the crucial premise of the second (i.e. that dualism is true). However, my argument will bring out a new challenge for theism: if the argument that I defend here is successful, then it follows that God acted arbitrarily in actualizing me over another self (or person). This is because multiple selves could have served the causal role that I do. But then why pick me over someone else? What could possibly ground this choice?

In its barest form, my argument is that physicalism entails that everything that exists is at least minimally supervenient, but selves are not minimally supervenient. Hence physicalism is false. Further, since selves are not minimally supervenient, it follows that they are non-physical. To show that selves are not minimally supervenient, I argue that they cannot be functionally reduced because it is possible for multiple selves to play the same causal role in the world.

One objection that I have pondering recently is that Twin Perry and I do not have identical causal roles because of our differing spatial locations. That is, Twin Perry’s causal role is (slightly) different than mine because he is causally related to Earth in a way that I am not, and I am causally related to Twin Earth in a way that he is not. While I’m not convinced that this difficulty is insurmountable (it is not clear to me that these differences are relevant given my definition of the self), we could tweak the argument to get around this objection as follows. First, note that Twin Earth and Twin Perry are possible. Second, note that this entails that Twin Perry can cause the same actions as I do—Twin Perry and I have overlapping causal powers. Lastly, note that this entails that my causal role does not point only to me, for Twin Perry could cause the same actions—play the same causal role—as I do. Hence Twin Perry and I may be inverted, and the objection mentioned above is rendered irrelevant.

Thank you for the paper. As someone who is skeptical that our reasonable modal intuitions extend very far, I appreciate your attempt to formulate an argument that does not turn on modal intuitions that might not be commonly shared.

However, I have some questions about your principle “A Picture of Observation.” First, a clarificatory question. You preface the principle as a “picture of how our modal beliefs may be grounded.” I assume you have in mind epistemically grounded or justified or reasonable or the like. But I was not sure what exact role this principle played in the argument. Is this a general claim that you think can be used by philosophers do defend the modal claims that people make in general? Do you think it underwrites very particular modal claims? Does the principle establishes the epistemological status of modal beliefs (i.e. identify them as empirical vs. non-empirical)? Anyway, it was not obvious how exactly you wanted to use the principle to me, though perhaps that is my own fault.

Second, I was wondering about (b) of the principle. For ease, I think it can be separated into two parts:

(b)(1) S knows that O obtains at time t, then it is possible that O obtains at tn (where tn, I assume, is some time)

(b)(2) S knows that O obtains at time t, then it is possible that O*–which is micro-physically identical to O—obtains simultaneously with O.

(I assume that this is how I’m to read the scope of ‘and’ in (b). Please correct me if wrong.)

Now (b)(1) strikes me as plausible and true. But it does not strike me as empirical or observational. It strikes me as a necessary, apriori truth that can be inferred from more basic such truths (e.g. if S knows that O obtains at t, then O obtains at t; if O obtains at t, then it is possible that O obtains at a time).

However, I’m less confident about (b)(2). First, depending on how exactly you intend to use the principle, it seems that (b)(2) might not have anything to do with knowledge. It seems what’s really key for your argument is a “duplication” principle, whereby if some macroscopic object exists, then it is possible that some other macroscopic object exists that has the same “micro-physical structure” or is “micro-physical identical.” Second, I’m not sure exactly what you have in mind by “micro-physically identical.” (From what I can tell, the phrase is first used in that principle.) So I find it hard to evaluate. But, finally, not matter how we spell out “micro-physically identical,” this principle does not strike me as an obvious observational claim (akin to there’s a rose on my desk). Rather, it strikes me as a claim that, if justified or grounded, would turn on being an application of some more general theory about structures of space-time. After all, I take it that even seemingly perfectly duplicated (say) coffee mugs coming off a factory line are not truly “micro-physical identical.” So the principle does not seem to be justified by appealing to our familiarity with similar looking macroscopic objects. However, my worry is that I simply don’t know enough about the structures of space-time to know that something like (b)(2) or some successor “duplication” principle is plausible. Could you perhaps say more as to why you think (b)(2) is plausible or how you imagine one might justify it?

Thanks for the comment. My argument does not necessarily (or even explicitly) rely on the principle. In fact, while I think that the principle (or something like it) is true and can provide epistemic justification for modal beliefs, it might have been better for me to omit it. That it was not clear to you how I meant to use the principle is my fault – I did not explicitly incorporate it.

Your reading of (b) is correct. The reason I classify my knowledge as empirical (in regards to (b)(1) and (b)(2)) is that I have observed O at t1 and O at tn, and that I have observed that O and O* can simultaneously obtain (e.g. I have observed two artifacts that are, by all visual accounts, micro-physically identical*). It looks like you disagree though: you suggest that I cannot be confident that I have observed two micro-physically identical organizations of matter. I think your worry has some force – on reflection, it is not obvious that I have observed that O and O* are identical.** But perhaps we can infer from observing O that O* could obtain simultaneously? O seems like pretty good evidence of this. Granted, this inference would not be strictly empirical, but perhaps it would be near enough.

*By micro-physically I have in mind something like a molecule by molecule identical object – same type of material and same structure (like Chalmers’ zombie).

**That said, it looks like my evidence for a cup C at t1 being identical to C at t5 (t5 is five seconds after t1) is equal to my evidence for C and C* (C* is a seemingly micro-physically identical cup that is exists simultaneously with C) obtaining simultaneously.

This is an interesting line of thought. I wonder, though, if the problem you indicate at the end of your comments above might be more serious than you think. MS only requires that the physical and the mental co-vary. However, the existence of Twin Earth, its relative location to Earth, etc., are all physical facts. So in the case you describe there is a physical difference.

Now, since Perry and Twin Perry are intrinsic duplicates, the case shows that we get a failure of local supervenience—that is, we can have two different people where the local physical facts are the same. However, many physicalists are externalists about mental states, so they might not be bothered by this. Maybe they should be, maybe not. It’s something to look into.

The same can be said for causal role. Presumably there’s a causal explanation for Earth ending up over here and Twin Earth over there, so there’s a difference in causal history, and physicalists often appeal to causal history to individuate mental states, determine their content, etc. So, again, the local causal role is the same, and we might have thought that was what really mattered, but there is nevertheless a difference in causal role.

Thanks for the comment. I think that you’re right that this is a serious worry. One possible way to circumvent the problem is if we allow that causal roles are defined in themselves, not by their history. Then the historical causal objection might be irrelevant due to the fact one’s causal role is independent of one’s historical origins (to put it roughly).* That said, I’m not sure how many physicalists (or functionalists) would affirm this type of causal role in regards to selves/persons. I will have to think about this more – this may force me to eliminate Twin Earth and focus on the possible solution I suggested in my introductory comments.

*Put differently, my total causal role can be understood abstractly (sort of like a computer program), and as such we can see that it can be implemented in multiple locations.

I’m not a metaphysician, so I feel a bit outside of my comfort zone. But here are a couple thoughts for your consideration.

Your argument seems to depend on the following key claim: for anything that I can observe as existing, I have empirically grounded knowledge that an intrinsic duplicate could exist simultaneously with it. Call it K. When I read your paper, moreover, I took your principle “A Picture of Observation” and your candle example to be attempts to defend or explain K. So I was surprised to read your reply to Tim Perrine just above, where you say that you are not relying on this principle at all. If not, then what reason have you offered us to accept K?

That issue aside, I must say that I am not particularly convinced that your principle, or your candle example, or K is true. Perhaps this is just another place in philosophy where reasonable people agree to disagree. However, notice that, in this way, your argument does in fact depend on a controversial modal intuition: one about the plausibility of a certain principle or claim K about modal knowledge or justified belief. You are right when you say that, if the principle/K is true, then we don’t need to appeal to the controversial metaphysical modal intuitions that plagued previous arguments. But whether or not your principle/K is true seems to itself depend on a controversial epistemological modal intuition nonetheless. This is not hard-hitting criticism of your argument, but it does push against the way you bill its value to the literature.

Finally, two very small side comments:

You say “Twin Earth is an organization of matter that is micro-physically structured exactly like Earth,” but that’s false. As discovered in earlier expeditions, the watery substance in Twin Earth is not composed of H2O, but of the very different XYZ instead. Jokes aside, there’s something odd about using an incredibly famous thought experiment for your purposes in this paper when one of its most famous deplyments is incompatible with your use here. At the very least, you should acknowledge this on a footnote; tell us you are borrowing the catchy name, but modifying a key aspect of the original idea. There’s no problem with that.

On footnote 6, you seem to equate Derek Parfit’s view on personal identity with “merely logically possible” skeptical hypothesis. I think that’s quite a bit uncharitable given his *extensive* defense of it. To imply that all of that work amounts to nothing more than simply pointing out that some non serious possibility is logically possible is to invite your readers to suppose you haven’t really read it. You are right that very many people are not moved by his arguments, which means that your argument is relevant to very many people. No need for the Parfit put-down.

Thanks for the comment! I will reply in order of the issues that you raised.

In response to Tim Perrine, I said that the argument does not explicitly or necessarily rely on the principle I mentioned (A Picture of Observation). However, I did not mean to claim that the argument does not rely on the candle example below it – I do not take the candle example and the principle to be synonymous. (I do not think that I was clear about this, so this confusion is my fault.) I think that the candle example is a paradigmatic case of modal knowledge, and that serves as our grounding for K (or something like K). In other words, I’m adopting the particularist approach to modal knowledge, and going from there.

You say that my argument relies on K which is controversial (and this is so even given my particularist approach to modal knowledge). I suppose that’s true – though, K seems less controversial than most modal premises. But we could put K aside for the moment, and say that it ultimately rests on the possibility of Twin Earth (though, see the final paragraph for in the above introductory section for an alternative way to state the argument). Is this modal claim highly controversial? I do not know. But it is certainly less outlandish than other modal premises. (That said, framing the argument this way takes away the appeal of grounding the key modal premise empirically. So my point here is just that the modal premise is relatively weak.)

Your comments incline me to think that the least controversial route would be adopt the approach in the final paragraph of my introduction. (If I were do this, I could frame the argument in terms of causal powers, which, I think, are easier to ground empirically.) For I grant that K is a controversial thesis, and that not all people will accept the candle example as a paradigm case of modal knowledge. I’m going to have to think about this more though.

Point taken on Twin Earth and Parfit. I appreciate you pointing this out.

Thanks for posting!
It’s an interesting argument, but – in addition to other concerns – I get the impression it would prove too much, like substance dualism for a number of other things, other than selves.

For example, mirroring your argument on page 5, an argument might go:

“Let us consider a possible world that contains both Earth and Twin Earth simultaneously. In this world, both twin desk and your desk exist. However, since twin desk and your desk are micro-physically and functionally identical—the only difference between them is spatial location—it follows that your desk cannot be functionally reduced. This is because twin desk and your desk play the same causal role, which entails that they may be inverted without causing a behavioral (or microphysical) change. From this it follows that MS is false for desks, for there may be a change in the (alleged) supervenient stuff (i. e. your desk and twin desk) without there being a change in the subvenient stuff, i.e., the microphysical stuff. Hence twin desk and your desk are non-physical. We may generalize this conclusion: since desks are not supervenient, it follows that desks are non-physical. Furthermore, since desks are substances, they are non-physical substances— substance dualism is true of desks.”

Granted, you might take the route of eliminativism for desks, and deny that your desk is a substance. That would only require that you modify part of the argument. But maybe I missed something, and you have some other reply?
If the “behavioral” part is a problem (because, arguably, desks don’t “behave”), I think one might just point out that they play the same causal role, as your footnote suggests in the cases of you and Twin Perry. The argument doesn’t seem to depend on whether there is a behavior in a narrow sense of the term “behavior”.

Even if narrow-sense behavior is an issue (but I’m not sure why that would be so), then one can still make a parallel including anything that exhibits behavior in a narrow sense, from ants to twisters to computer viruses. Granted, you might go for a much narrower sense of “behavior”, but in that case, I would be inclined to ask for a definition of “behavior” (at least, an ostensive definition that allows me to grasp the concept), and also an argument showing why exhibiting behavior in that very narrow sense is relevant to the argument for substance dualism.

Thanks for the reply. That is a really interesting issue you raise. My inclination is to think that desks do not need to be functionally reduced, for they can be reduced (roughly) to the matter that they are. In other words, with desks we can go further than functional reduction. (I do not think this can be done with selves, so I do not think this reply is available to the physicalist.)

I’m not sure why the physicalist could not suggest reduction to specific matter, or maybe better to something involving causal history and matter (or simply be silent on the issue of reductions, without a commitment to a functional reduction).

A reduction doesn’t have to be precise to any arbitrary degree; there may be some room for vagueness in identity words or other factors that might prevent a precise reduction in terms of possible worlds, but the same seems to happen to desks and particles. For example, as you say, your desk “has existed for many years and has the property of being brown, the property of weighing over twenty pounds, and the property of being in my living room”. But your desk today doesn’t have exactly the same particles as it had 20 years ago. Due to erosion, it has lost many particles, and the weight has also changed in a minuscule proportion, but it’s still your desk.
So, if desks can be reduced to (roughly) to the matter that they are, perhaps the physicalist may suggest selves can be reduced (roughly too) to the matter they are, or to that matter plus their causal history, or to matter+causal history+function (i.e., a more complex reduction).

On a view involving causal history, Twin Perry and you cannot be exchanged, because your causal history (over here) and his causal history (over there, on Twin Earth) are very different. Similarly, on a view involving specific matter, the argument seems to be blocked. And it’s also blocked on a view where the reduction involves both specific matter and causal history.
Maybe you have arguments against such hypothetical reductions, but I think that would require giving arguments to cover them.

Another alternative seems to be a panpsychist view. For example, one might sketch the following simple panpsychist hypothesis:

PAN1: “Particles have, alongside physical properties, some mind-like properties (minimum phenomenal consciousness, or something like that). When they combine with each other, they might combine only the physical properties (e.g., probably, when they make a desk), or also some of the mind-like properties, resulting in a (much) more complex mind (e.g., when they make a functioning human brain). So, complex minds causally result from combinations of particles, and there are no souls or spirits beyond that”.

Let’s say Alice supports PAN1. She willingly grants that it’s possible that you and Twin Perry exist at the same time in a universe that works like ours, save for the problem of simultaneity in relativity. She further says that it’s epistemically possible that Twin Perry actually exists – if the universe is very big, then there are duplicates of all of us; in fact, she even says that for all she knows, there might actually even be infinitely many twins of each of us.

I don’t see how your argument might work against something like PAN1, so it seems to me a panpsychist (also) may avoid the dualist conclusion. Or do you think PAN1 is a form of substance dualism?

For my paper, I assumed the physicalist endorses functional reduction, so that’s why I didn’t really touch on this issue (in my experience, this is the most popular approach). Regarding your question as to why the physicalist can’t just reduce the self to a specific set of matter (+causal history +function). One reason I think that she cannot do this is that it commits her to two very strong and implausible modal claims: (1) that if *that* matter, *that* function, and *that* causal history obtain, then *necessarily* she obtains as well, and (2) that (a) she could not *possibly* exist while serving a different function (i.e. she could not perform any other actions than she does), and (b) she could not have existed at any different time (her causal history would be different). Both of these strike me as implausible, however, I recognize that some physicalists might not be bothered by these. (2)(a) strikes me as the most implausible consequence (though, you could always get rid of your function clause to avoid this). Anyway, these are my initial thoughts on this issue, do you find these modal commitments to be plausible? Or have I misunderstood you?

Regarding panpsychism: it strikes me anti-physicalist, but not really dualistic – unless you hold that the emergent mind is both a substance and has a contingent connection with matter. So I need more details to know how to respond to this.

(1) I don’t think she’s committed to (1). Perhaps, she would be committed to (1′): “Necessarily, if *that* matter, *that* function, and *that* causal history obtain, then she obtains as well”. But I don’t see why the necessity claim would be in the consequent.

In any case, I think the argument works against the reduction of your desk to the matter is made of, but not against the physicalist, for the following reasons:

1.1. I find (1) plausible myself, even if I don’t take a stance on whether physicalism is true (for all I know, maybe panpsychism is true, or maybe something else no one has come up with yet). I don’t think a physicalist would probably be bothered by (1).
1.2. The hypothesis that the physicalist is committed to (1) if she “reduces the self to a specific set of matter (+causal history +function)”, seems to be implicitly committed to the following principle:

P(1): If A is reduced to B, then “If B obtains, A necessarily obtains” is true.

The hypothesis that the physicalist is committed to (1′) if she “reduces the self to a specific set of matter (+causal history +function)”, seems to be implicitly committed to the following principle:

P(1′): If A is reduced to B, then “Necessarily, if B obtains, A obtains” is true.

But if either P(1) or P(1′) is true, then it’s not the case that the desk can be reduced to the matter it’s made of, since it’s clear that it’s possible that all of the matter that makes your desk exists, but your desk doesn’t. The matter possibly is making other things, like a bed, or chairs, or part of a car, or part of a planet, or some of the matter might be on one planet, some of the matter on another planet, and so on.
So, for any specific matter M1, the following statements are false: “Necessarily, if M1 obtains, your desk obtains”, and “If M1 obtains, then necessarily, your desk obtains” (the latter might be true as a material conditional if M1 doesn’t obtain, but that’s not relevant in this context).

I’m not sure how you would then reply the modification of your argument to make it an argument for desk dualism, given that desks cannot be reduced to the matter they’re made of.

(2) (a) I don’t think she’s committed to that, for two reasons:

2a.1. She can be a compatibilist about power and believe that “A could do X” does not entail “Possibly, A does X”.

2a.2. More importantly, I’m not sure why “function” would have to be interpreted in such a strong fashion. If it is interpreted like that (i.e., she necessarily performs the actions she actually performs), couldn’t you just object to the physicalist functional reduction on the ground that the physicalist is committed to the conclusion that she couldn’t possibly perform any actions other than what she does? That would seem to be a more direct argument than the argument in your paper.
On the other hand, if functional reduction does not commit the physicalist to that conclusion, I’m not sure why function+matter+causal history would.
Personally, I think any attempted reduction would be fuzzy, whether when applied to people or to desks. The physicalist might take that approach too, and say that our identity terms also track a complex combination of matter, causal history and function, but they’re also fuzzy and they don’t require *exactly* that matter, causal history and function – something similar will do. Further, she may hold that that is so when it comes to people, and when it comes to desks, or to anything else, so her hypothesis is general, not ad-hoc.

(2) (b) I don’t think she’s committed to that claim, because she can have the same causal history as she has in the actual world, but then – say – someone invents a way of preventing aging, and she gets to live for billions of years, so she also exists in the distant future.
What she may be committed to is the claim that it’s impossible that she existed in the past, save, perhaps, for some fuzziness. But I don’t think that’s problematic for the physicalist, and in fact it seems common among theists too.
For example, Alexander Pruss’s view ( https://alexanderpruss.blogspot.com.ar/2016/01/essentiality-of-origins-and-non.html ) entails that it’s impossible for any (human) person to have had other parents than the ones she had, and furthermore, that it’s impossible that a person exists if the past before she was born is different, even if very slightly so (a result the physicalist may avoid if she goes with the fuzziness hypothesis I mentioned).

In re: the panpsychist, she doesn’t hold that the mind is entirely emergent; rather, there is mind all the way down so to speak, but what in some way “emerges” is that the more complex mind causally results from the interactions of the particles, involving their own mind-like properties. But she’s silent on the issue of whether the emergent mind is both a substance and has a contingent connection with matter (I’m not sure it matters, but let’s say her views entail there is no persistence of the self after death, since the particles are no longer interacting in the right way).

1.1 Fair enough.
1.2 Right, I should make it a little more specific, citing more than there mere existence of the matter (perhaps taking into account structure, origins, etc.). I think I originally worded this quite sloppily.
2(a).2 I’m inclined to think that the physicalist – if she is making use of functional reduction – would need to reduce it in the strong sense. Perhaps you’re right that that is a more direct route to go.
Fuzziness: if one holds that identity has fuzzy boundaries you’re right that one can take this route. However, even once fuzziness comes into play, it isn’t clear to me how this helps her: does it make an inversion not possible? Does it make a (non-functional) reduction more plausible? Not that I can see (though, perhaps I cannot see very far!).

Essentiality of origins: you’re right that lots of people find that plausible, so I was too quick above. However, if we are assuming that a functional reduction is what the physicalist is wanting to do, then it seems to me that origins are not essential, for one’s causal role is (in a significant sense) independent of her origins. (This analogy isn’t perfect, but…: we can have the same program on two different computers. If we think of programs in a functional sense, then we have two functionally identical programs despite them having different origins (i.e. used this disc to install mind, but you used that disc to install yours.))

Panpsychism: if that’s her position, then it seems to me to be anti-physicalist but not a form of substance dualism. So I do not think my argument will work against a panpsychist.

No problem, and thanks for replying.
I misspoke too. I meant to say that I find (1′) plausible, that the physicalist would probably not be bothered by (1′), etc., rather than (1). But I guess you got that already.

1.2. If you take into account account not only matter but structure, origins, etc., the physicalist can argue that whatever reduction works for desks, a similar one may well work for humans.

2(a).2. Okay, in that case, it seems to me that functional reduction (or functional + causal history + matter, or anything containing function) should be rejected in a straightforward manner, because it’s clear one possible does something one actually doesn’t. Similarly, anything containing function should be rejected for desks. But the physicalist can still go with a “partners in innocence” argument, and say that whatever reduction works for desks (considering matter, structure, origins, etc.), it works for people.
Regarding the fuzziness issue, the idea would be also to mirror the desk case, so as to find a partner in innocence.
A potential objection is to say that if a person dies, the no longer exist despite the existence of the matter, origin, structure, etc., but a physicalist might reply in two ways:

A: The relevant structure in the case of people is no longer present when a person dies, and of course there are physical differences between dead people and living people.
B: Instead of desks, make the analogy with, say, a laptop computer, or a computer virus, or an actual virus.

In re: essentiality of origins, I was replying to your point about a physicalist that reduces the self to function+causal history+matter. You suggested she’d be committed to the view that “she could not have existed at any different time (her causal history would be different)”. I was saying that the view that it’s not possible that she existed in the past is also shared by many non-physicalists who believe in essentiality of origins, so it wasn’t an unusual commitment.

1.2 I think you’re right. While I don’t think that this move would be plausible to make, I need to elaborate on why I think this is so – at least for this argument to work on the non-functional reductive physicalist.

2(a).2 Just to be clear, I’m following Kim’s notion of functional reduction on this. However, you have given me a lot to think about here in regards to whether such a characterization will satisfy the physicalist – she may need a more robust version of functional reduction, and the more robust version could help her circumvent the argument. (If you have any paper recommendations on this issue, let me know.)

Regarding 2(a).2., thanks for the clarification. I know Kim’s model has been criticized on a number of grounds, but I’m not familiar with any better models. That may well be because I’m not familiar enough with the matter, but still, even if there are better models, personally I don’t think a functional reduction is promising (at the very least, not without adding other components to function).